Method of forming sandwich materials

ABSTRACT

The invention provides a method of forming sandwich materials. The area subjected to forming stresses is heated locally in such manner that the strength properties of the skin metals governing the permissible stress limits in that area are constantly monitored without the structure as a whole being subjected to oxidation.

United States Patent [1 1 Fremont et al.

[ Dec. 2, 1975 METHOD OF FORMING SANDWICH MATERIALS [75] Inventors:Maurice, Henri, Louis Fremont,

Massy; Jean-Francois Denis, Lesigny; Serve, Yvan Dzalba-Lyndis,Villejuif, all of France [73] Assignee: Societe Nationale IndustrielleAerospatiale, Paris, France [22] Filed: July 8, 1974 [21] Appl No.:486,428

[30] Foreign Application Priority Data July 10, 1973 France 73.25292[52] US. Cl 72/l28;'72/342 [51] Int. Cl. B21D 47/00 [58] Field of Search72/128, 342; 29/455 LM [56] References Cited UNITED STATES PATENTS622,282 4/1899 Smith 72/128 2.737224 3/1956 Jones 72/342 3.788.117l/l974 Chester et al 29/455 FOREIGN PATENTS OR APPLICATIONS 6 742 1965Japan 72/128 Primary Ev\'aminerLowell A. Larson Attorney, Agent, orFirmKarl W. Flocks [57] ABSTRACT The invention provides a method offorming sandwich materials. The area subjected to forming stresses isvheated locally in such manner that the strength properties of the skinmetals governing the permissible stress limits in that area areconstantly monitored without the structure as a whole being subjected tooxidation.

4 Claims, 13 Drawing Figures US. Patent Dec. 2, 1975 Sheet 1 of43,922,899

X r y FIG] Q? T f 7' -L- fy Wi y 2 C I 42 c 1 FIG) FIG! Sheet 2 of 43,922,899

FIGB

US. Patent Dec. 2, 1975 Sheet 3 of4 3,922,899

U.S. Patent Dec. 2, 1975 Sheet 4 of4 3,922,899

1 ox /DAT/ON METHOD OF FORMING SANDWICH MATERIALS The present inventionrelates to a method of forming sandwich materials.

The difficulties of forming the so-called sandwich materials used in theaerospace industry, in which two thin sheets of metal are maintained inmutually spaced relationship by a spacing element or web, are wellknown. For example, this spacing element may be either a corrugatedelement as used in the materials employed by the Applicant under thetrade-name Norsial", or a honeycomb type of element as used in thematerials employed by the Applicant under the tradename Nida".

These difficulties are mainly due to the fact that it is not alwayspossible to impart shape to such materials directly or to form them fromflat panels because local buckling phenomena tend to cause thecompressed face (or skin) to wrinkle while the stretched face (or skin)is subjected to high tensile stresses.

However, when the neutral or zero-stress fibre passes midway through thethickness of the material, the ten sile and compressive stresses in thetwo skins are of equal and opposite values, and therefore a knowledge ofthese values can determine the magnitude of the deformation which can beapplied to the panel.

Indeed, by applying appropriate overall heating it is even possible toso modify the principal characteristics of materials as to make itpossible to increase the obtainable extent of deformation. However,although this heating method is applicable to many metals such asstainless steel, nickel, etc., this is not so in the case of many otherspecial metals like titanium and its alloys, molybdenum, magnesium,beryllium and their alloys, which metals do not stand up well toprotracted overall heating because of their great sensitivity tooxidation.

Obviously, protection against oxidation can be obtained by carrying outsuch heating in an evacuated enclosure or in a neutral atmosphere, butthis is an inconvenient and costly solution when the items to be formedare of considerable size.

The present invention provides a method of forming sandwich materials ofthe above-mentioned kind, made of oxidizable metals, that overcomes thedrawbacks of the prior art.

In the method according to this invention, the area subjected to formingstresses is heated locally in such manner that the strength propertiesof the skin metals governing the permissible stress limits in that areaare constantly monitored without the structure as a whole beingsubjected to oxidation.

The description which follows with reference to the accompanyingnon-limitative exemplary drawings will give a clear understanding of howthe invention can be carried into practice.

In the drawings:

FIGS. 1 and 2 are sectional side elevation views of Norsial and Nidatype sandwich panels, respectively, in which the neutral fibre (x-y)lies in the median plane through the thickness of the panel;

FIGS. 3 and 4 are illustrations corresponding to FIGS. 1 and 2, for thecase where the neutral fibre (x-y) is offset and does not lie in themedian plane through the thickness of the panel;

FIGS. 5 and 6 are diagrammatic illustrations showing the stressdistribution when a sandwich panel is subjected to bending and tensionrespectively;

FIG. 7 is a graph for illustrating the local buckling phenomenon in asandwich panel;

FIG. 8 is a graph showing the effect of oxidation on a metal oxidizablein free air, as a function of time and temperature; I

FIG. 9 diagrammatically illustrates a first possible arrangement forperforming the subject method of this invention;

FIG. 10 is a graph in which the strength properties of a titanium alloyare plotted against temperature;

FIG. 11 is a micrographic image of the effect of oxidation underprotracted heat in an assembly of titanium alloy sheets;

FIG. 12 is a micrographic image of the effect of oxidation under heat ina titanium alloy assembly treated in accordance with the presentinvention; and

FIG. 13 diagrammatically illustrates an alternative possible arrangementfor performing the subject method of this invention.

One of the difficulties encountered in operations for forming Norsial orNida type sandwich panels stems from the fact that it is almostmandatory to resort to a bending load F (see FIG. 5). The effect of suchbending is to produce a tensile stress T, on the stretched face (orskin) 1 and a compressive stress C, on the compressed face (or skin) 2,but these stresses will be of equal magnitude provided that the neutralfibre is equidistant from the external faces of the panel. (For greaterclarity, FIGS. 1 and 2 represent equal stresses T and C for the casewhere the neutral fibre x-y lies at equal distances r from the externalfaces 1 and 2 of Norsial and Nida type panels respectively, and FIGS. 3and 4 represent unequal stresses T and C for the case where the neutralfibre x-y is offset in relation to the median planes of said panels).

The strain in each case can readily be calculated from the elementaryformula:

where 0' is the strain,

M the bending moment,

I the inertia of the material and r the distance of the external faces(or skins) from the neutral fibre x-y.

In practice, the tensile stress T does not present a major drawbackprovided that the metal possesses adequate capacity for elongation. Onthe other hand,- the compressive stress in the compressed skin 2 soonresults in a local buckling effect which produces increasinglyaccentuated wrinkling. This phenomenon will be seen to be virtuallyinevitable from and examination of the graph in FIG. 7, in which E isthe deformation,

0' the strain,

0', the strain beyond which local buckling of the compressed skinoccurs,

and 0 the strain at the elastic limit'of the metal for an 0.2%elongation.

If it is desired to deform the material permanently, it is indispensableto greatly exceed the metals elastic limit at 0.2% elongation, a limitwhich is usually greater than thestrain 0', producing the local bucklingphenomenon.

This drawback can often be avoided, in particular through the use ofso-called stretch-forming methods in which a tensile force T is exertedon the panel prior to deformation by bending, as shown in FIG. 6. Thisstress T comes in deduction of the compressive stress (C,- T thatappears in the internal skin 2 and sets back the onset of the localbuckling phenomenon correspondingly. Contrariwise, it is added (T, T tothe tensile stress in the outer skin 1 and therefore limits thedeformation possibilities by reason of the high stresses involved, whichcould result in rupturing of the stretched skin 1.

As is well-known, the forming of oxidizable parts, especially titaniumor titanium alloy parts, is facilitated if it can be carried out underheat; for in addition to the fact that the rise in temperature improvesthe basic strength characteristics, it enables the elastic restoring orresilience effect, which is particularly strong in such materials whenthey are cold, to be avoided. Unfortunately however, in order to beeffective, the temperature must be high (in excess of 600C) and it iswellknown that at such temperatures all these oxidizable metals are veryseriously contaminated by the atmosphere, resulting in a notablydiminished resisting section and in the appearance of oxidized cracks.In consequence, a conventional forming operation would require a fairlylong time during which oxidation and contamination would develop by theprocess shown in FIG. 8, in which the temperature in degrees centigradeis plotted along the X-axis and the oxidization depth in millimetersalong the Y-axis. Curves I, II, III and IV in FIG. 8 correspond toheating times of /2 hour, 1 hour, 2 hours and 4 hours respectively, andit may be noted that the depth of oxidization as a function oftemperature and time does indeed vary between about 0.03 mm and 1.1 mm.

When it is remembered that the skins of sandwich panels to be formed areno more than a few tenths and sometimes a few hundredths of a millimeterthick as a rule, it will be clear that with such an operation therewould be virtually no sound" metal left, making it impossible to treatsuch materials by conventional openair methods.

The solution consisting in placing the parts to be shaped in anevacuated enclosure or in a neutral atmosphere, though valid for smallparts, would be difficult to apply in the case of items of substantialsize.

The forming methodaccording to this invention enables all the drawbacksmentioned hereinbefore to be overcome.

The subject method of the invention firstly allows of very substantiallydelaying the onset of local buckling of the compressed inner skin andtherefore increases the forming possibilities for any given panel, andsecondly authorizes the forming of oxidizable parts, and especiallytitanium alloy parts, in very short times during which oxidation hasvery little chance to develop, even without gaseous protection. Itshould be noted that such short-time forming by no means implies highdeformation speeds, but quite the opposite, thereby enabling advantageto be taken of the relaxation phenomena well-known in metallurgy.

Essentially, this method is characterized by the fact that it consistsin heating the skins of a metal sandwich panellocally and differentiallyin such manner as to ensure that the tensile and compression stressesengendered therein during forming are optimal having regard for thestrength characteristics of the metals in question.

In accordance with further teachings of this invention:

the local heating zone is proximate the instantaneous deformation zone;

- the forming is carried out by applying the panel against a rotatableformer and by providing local heating means in immediate proximity tothe points at which the panel to be formed is tangential to said former;

and in the specific case of titanium and its alloys, the temperature ofthe hotter skin is approximately 770C, thereby providing a modulus ofelasticity of about 6200 hb, and the temperature of the colder" skin isapproximately 480C, thereby providing a modulus of elasticity in theregion of 8500 hb.

The invention likewise relates to arrangements and means for performingthe said method, which arrangements are described hereinbelow forexemplary purposes with reference to FIGS. 9 and 13.

Reference is first had to FIG. 9, which illustrates a first way ofperforming the subject method of this invention.

The panel to be formed 3 is placed with its inner face 7 against anyconvenient rotating former 4. Local heating means 5, such as an iodinevapour or infrared-tube radiator heats the metal locally on the outerskin 6, in proximity to the line of instantaneous deformation, that isto say at the points where the panel to be formed is tangential to theformer. A roller-type restraining device 8 prevents the panel fromlifting, and possible tensioning means 9 exert a traction on the panelin order to produce additional overall stretching.

The surface of former 4 can be coated at 10 with insulating substancessuch as asbestos or melted ceramic, or alternatively with metals likecopper or aluminium so that the good heat-conducting properties thereofmay ensure optimum heat distribution through the panel.

Using the subject method of this invention and the above-describedarrangement, the Applicant has been able to make a circular cylinderwith an inner diameter of I00 mm, made of welded Norsial sandwichmaterial consisting of a corrugated web in 0.15 mm-thick sheet withcorrugations pitched at 6 mm and two 0.3 mmthick skins in TA6V4 titaniumalloy (6% of aluminium and 4% of vanadium). The panel had a totalthickness of 4.3 mm and the wrapping rate was 6 mm per minute.

The local heating was providedby an iodine-vapour radiator with a linearheating zone, positioned in such manner that the area heated on theouter skin 6 was a generatrix of the cylinder approximately 3 mm wide.

The temperature noted on the heated outer skin was 770C and that of theinner skin in contact with the former (which was made of insulatingmaterial) was 480C.

The curves in FIG. 10 (obtained by plotting the temperature along theX-axis and the strength characteristics (r E and A as hereinbelowdefined along the Y- axis) give the values of these strengthcharacteristics in the case of TA6V4 titanium alloy sheet 0.3 mm thick.It may be noted from FIG. 10, where 0 is the tensile strength at theconventional elastic limit for 0.2% elongation. E is Youngs modulus ofelasticity, and A% is the ultimate elongation, that the elastic limitand Youngs modulus decrease with rising temperature and that,conversely, the permissible elongation increases considerably, albeitafter a small transitory decrease.

Thus when the area of the sandwich material being formed at any giveninstant is uniformly heated, the surface of the neutral fibres extendsmidway along the panel by reason of the thermal symmetry achieved, andthe tensile and compressive stresses are accordingly equal in absolutevalue.

When however, in accordance with this invention, there is thermalasymmetry by reason of preferential heating of the outer skin, thelatters modulus of elasticity becomes less than that of the inner skinand the surface of the neutral fibres shifts towards the compressed skinand the tensile and compressive stresses are no longer equal.

For instance, when the outer hot skin is tensioned to a modulus ofelasticity of 6200 hb at 770C, the inner cooled skin is compressed to8500 hb at 480C and the downward shift of the neutral-fibre surface thencorresponds to the ratio 8500 hb/6200 hb or 1.37, as schematicallyillustrated in FIGS. 3 and 4.

The compression in the inner skin is thus considerably less than in thecase of uniform stress referred to precedingly, and this skinfurthermore possesses high rigidity (8500 hb). It is accordingly lightlystressed and will withstand buckling, and the unacceptable drawbacks oflocal buckling are thus eliminated. Another consequence of the shiftedposition of the neutral fibres is to increase the tension in the outerskin, or in other words the elongation required to achieve a permanentset. However, this increase is offset by the fact that that face is athigh temperature, and that at that temperature the permissibleelongation, which is then 30% as shown in FIG. 10, is over-abundant andeasily covers most foreseeable contingencies for the sandwich materialsconsidered by the present invention.

FIG. 11 shows for exemplary purposes, in the case of 0.27 mm and 0.14 mmthick TA6V4 titanium alloy, the corrosion effect obtained in air duringprotracted heating at 800C. The micrographic image shows clearly, withits magnification of 125 times, that the oxidized layer is very thick.

Conversely, the micrograph image in FIG. 12 (which shows the joiningarea of other such sheet metals of similar nature forming a sandwichpanel processed by the subject method of this invention) clearly revealsthe thinness of the oxidized layer even though the image is magnified340 times in this case.

By way of an alternative arrangement for obtaining a panel in accordancewith this invention, FIG. 13 shows an arrangement similar to that inFIG. 9, in which the rotating former 11 is non-cylindrical and the panel3 is heated by a radiator 5 having infrared tubes and is restrained by athrust roller 12.

All the aforementioned embodiments, regardless of whether they involvethe use of a cylindrical or noncylindrical mandrel, employ the samemethod of this invention, which provides for suitably adapting localstresses in materials by locally heating the two skins of a Norsial" orNida type sandwich panel symmetrically.

It goes without saying that the present invention has been described fornon-limitative exemplary purposes only and that changes andsubstitutions may be made without departing from the scope of theinvention as defined in the appended claims.

We claim:

1. A method of forming a metallic panel sandwich comprising two thinmetal sheets maintained in mutually spaced relationship by a spacerelement of honeycomb structure, corrugated elements, or the like.wherein the said method includes:

a. placing one sheet of a metallic panel sandwich against a rotatingformer of rounded surface;

b. applying radiant heat to the immediate proximity of points where saidmetallic panel sandwich is tangential to said rotating former whereby alocalized and differential heating effect is provided in the two thinsheets passing thereby; and

c. simultaneously rotating said former to thereby effect formation ofsaid metallic panel sandwich.

2. The method of claim 1 wherein said radiant heat is obtained from aniodine vapour or infrared-tube radia- 3. The method of claim 1 whereinsaid heat is supplied from a source spaced from and not contacting saidmetallic panel sandwich.

4. The method of claim 1 wherein said application of radiant heat is setto provide a temperature of 770C in one of said two sheets and atemperature of 480C in the other of said two sheets, where said twosheets are of titanium or titanium alloys.

UNITED STATES PATENT OFFICE QERTEFICATE 0F CORRECTION Patent 3,922,899Dated December 2, 1975 Inventor) Maurice Henri Louis Fremont,Jean-Francois Denis Serge Yvan Dzalba-Lyndis It is certified that errorappears in the above-identified patent and that said Letters Patent arehereby corrected as shown below:

In column 1, the third inventor s first name should read: Serge Signedand Sealzd this thirtieth D f March 1976 [SEAL] Arrest:

RUTH C. MASON C. MARSHALL DANN Alresling Officer Commissioner uj'Patemsand Trademarks UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTIONPatent m 3,922,899 Dated December 2, 1975 Inventor) Maurice Henri LouisFremont, Jean-Francois Denis,

Serge Yvan Dzalba-Lyndis It is certified that error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

In column 1, the third inventor s first name should read; Serge Signcdand Scaled thisthirtieth D f March 1976 [SEAL] Arrest:

RUTH C. MASON C. MARSHALL DANN Altesling Officer Commissioner nfPatentsand Trademarks

1. A method of forming a metallic panel sandwich comprising two thinmetal sheets maintained in mutually spaced relationship by a spacerelement of honeycomb structure, corrugated elements, or the like,wherein the said method includes: a. placing one sheet of a metallicpanel sandwich against a rotating former of rounded surface; b. applyingradiant heat to the immediate proximity of points where said metallicpanel sandwich is tangential to said rotating former whereby a localizedand differential heating effect is provided in the two thin sheetspassing thereby; and c. simultaneously rotating said former to therebyeffect formation of said metallic panel sandwich.
 2. The method of claim1 wherein said radiant heat is obtained from an iodine vapour orinfrared-tube radiator.
 3. The method of claim 1 wherein said heat issupplied from a source spaced from and not contacting said metallicpanel sandwich.
 4. The method of claim 1 wherein said application ofradiant heat is set to provide a temperature of 770*C in one of said twosheets and a temperature of 480*C in the other of said two sheets, wheresaid two sheets are of titanium or titanium alloys.